Aerogel Research at LBL: From the Lab to the Marketplace

Summer 1991

By Jeffery Kahn (jbkahn@lbl.gov)

Aerogels produced at LBL are 96-percent air mixed with a wispy matrix of silica. Despite their lack of substance, these materials are the world's
best solid insulator, transmitting only one hundredth the heat of normal
glass. See the Lab's Microstructured
Materials Group website for additional information.

On
first sight, silica aerogels cause most people to do a
double take. Observers perceive a ghost-like substance, what looks
like fog that somehow has been molded into a distinct form yet fog
which is encased by no evident means. Almost like solid smoke, an
aerogel resembles a hologram, appearing to be a projection rather
than a solid object.

Aerogels are advanced materials yet also are literally next to
nothing. They consist of more than 96 percent air. The remaining
four percent is a wispy matrix of silica (silicon dioxide), a
principal raw material for glass. Aerogels, consequently, are one
of the lightest weight solids ever conceived.

Arlon Hunt was working in LBL's solar energy and energy
conversion research program in 1981 when he first saw an aerogel
which had been brought to the Laboratory by a visiting Swedish
professor. Hunt says that immediately, he was fascinated by the
material and its manifest possibilities.

"I was intrigued by how lightweight, transparent, and
amazingly porous the stuff was," recalled Hunt. "Porous materials
scatter light and almost always are opaque or whitish. The near
transparency of the material implied extremely fine pore structure.
Later, I found out just how fine."

As Hunt learned of the unique thermal, optical, and acoustical
properties of aerogels, he became further intrigued. Since 1982,
the Applied Science Division researcher has explored fundamental
questions about the properties of aerogels and developed processes
for creating thermally and optically-enhanced versions. Over the
past decade, Hunt also has evaluated aerogels for many applications
and developed chemical production methods suitable for commercial
manufacturing.

Made of inexpensive silica, aerogels can be fabricated in
slabs, pellets, or most any shape desirable and have a range of
potential uses. By mass or by volume, silica aerogels are the best
solid insulator ever discovered. Aerogels transmit heat only one
hundredth as well as normal density glass. Sandwiched between two
layers of glass, transparent compositions of aerogels make possible
double-pane windows with high thermal resistance. Aerogels alone,
however, could not be used as windows because the foam-like
material easily crumbles into powder. Even if they were not
pulverized by the impact of a bird, after the first rain they would
turn to sludge and ooze down the side of the house.

Aerogels are a more efficient, lighter-weight, and less bulky
form of insulation than the polyurethane foam currently used to
insulate refrigerators, refrigerated vehicles, and containers. And,
they have another critical advantage over foam. Foams are blown
into refrigerator walls by chlorofluorocarbon (CFC) propellants,
the chemical that is the chief cause of the depletion of the
earth's stratospheric ozone layer. The ozone layer shields life on
Earth from ultraviolet light, a cause of human skin cancer.
According to the Environmental Protection Agency, 4.5 to 5 percent
of the ozone shield over the United States was depleted over the
last decade. Based on the current levels of ultraviolet exposure,
the agency projects that more than 12 million Americans will
develop skin cancer and more than 200,000 will die of the disease
over the next 50 years.

Replacing chlorofluorocarbon-propelled refrigerant foams with
aerogels could help reduce this toll. Exchanging refrigerant foams
with aerogels reportedly would reduce CFC emissions in the U.S. by
16 million pounds per year.

Aside from their insulating properties, aerogels have other
promising characteristics. Sound is impeded in its passage through
an aerogel, slowed to a speed of 100 to 300 meters per second. This
could be exploited in a number of ways, as for example, improving
the accuracy and reducing the energy demand of the ultrasonic
devices used to gauge distances in autofocus cameras and robotic
systems. A layer of aerogel on a camera's ceramic piezoelectric
transducer could considerably improve the efficiency with which it
generates ultrasonic waves.

Aerogels also have a number of novel applications. Currently,
they are components of Cerenkov radiation detectors used in high-
energy physics research at CERN near Geneva, Switzerland. Another
scientific application currently under consideration involves
utilizing aerogels in space like a soft, spongy net to capture
fast-moving micrometeroids without damaging them.

The new generation of aerogels that Hunt is creating is based
on the groundwork laid down in the 1930s by Stanford University's
Steven Kistler. Kistler worked with gels and in a 1932 paper
published in Nature, resolved many of the basic questions about
this odd form of matter. Kistler showed that a gel is an open
structure composed of a matrix of solid pore walls and a liquid
fill. Subsequently, he invented a way to dry a gel of its liquid
contents without collapsing or shrinking it. Kistler called his new
material an aerogel.

Aerogels -- the name pays tribute to the near paradoxical
accomplishment of creating a hybrid between a gel and thin air --
are not known to exist in nature. Jellyfish are a non-manmade
example of a gel, and a dead jellyfish washed up on the beach and
baked by the sun is an illustration of what happens to a gel when
it is dried in nature. Unlike aerogels which start out as a gel and
do not lose volume as they dry out, a dead, desiccated jellyfish
ultimately shrinks to 10 percent of its former size.

After Kistler brought aerogels into the world, they remained
a forgotten phenomenon for three decades. Briefly, they reemerged
in the scientific literature in the 1960s but aerogels were not
fully resurrected as an object of significant scientific curiosity
until the 1980s. That was when Hunt first encountered aerogels and
immediately, he saw their potential. Hunt also realized that
Kistler's aerogels had drawbacks. As they were at the time,
aerogels were commercially worthless.

Aerogels were cloudy rather than totally transparent. Before
they could be used in double-pane windows or skylights, clarity had
to be improved. Aerogels were splendid insulators but in order for
them to become a cost-effective alternative to existing products,
they had to be made even more thermally resistant. From the
standpoint of fabrication, several obstacles emerged. The extant
chemistry and processing technology was too expensive and it was
potentially explosive. Finally, processing required toxic compounds
which presented yet another impediment. Taken together, a
formidable phalanx of technological barriers prevented aerogels
from making the leap from the laboratory to the consumer.

Over the past eight years, Hunt has confronted each of these
obstacles. Fundamental studies he has conducted have resulted in
applied advances toward resolving each of the major shortcomings.
Along the way, Hunt founded a private firm which has been licensed
by the Laboratory to manufacture and sell aerogels. Thermalux, L.P.
is the only U.S. aerogel firm and has set up a development-stage
pilot plant in Richmond, California. Currently, a Swedish company
that produces aerogels for use in radiation counters, is the only
other commercial aerogel manufacturer in the world.

Whether they are the commercial aerogels Thermalux is
fabricating for tests and assessment by the refrigeration industry
or the experimental compounds Hunt is producing in his laboratory,
all aerogels start out as a gel. A gel consists of chains of linked
particles or polymers permeated by a liquid. To transform a gel
into an aerogel, the liquid must be removed without collapsing the
solid framework. This is a tricky proposition.

The gel lattice consists of solid pore walls filled by a
liquid. When liquid is evacuated from a gel, normally surface
tension overwhelms the porous network, causing it to collapse. As
air replaces the liquid inside each pore, surface tension
inexorably pulls the sides of the pores together and the gel
shrinks.

Kistler discovered the secret to drying a gel without
collapsing it. He dried his gels at elevated temperatures and
pressures, transforming the liquid to a supercritical state wherein
there is no longer a distinction between a liquid and a gas. After
cranking up the temperature and pressure to create supercritical
conditions, pressure is slowly released. The supercritical fluid is
vented out of the gel matrix without any surface tension effects.
What remains is an aerogel that is more than 96 percent air.

Aerogels were exquisite structures but they were formulated
with a standard starting compound known to damage the cornea of the
eye. The toxic material, tetramethylorthosilicate (TMOS), had been
introduced in the 1960s as a means of reducing the preparation time
for aerogels from several weeks to a few hours. Hunt and his
colleagues Rick Russo, Mike Rubin, Kevin Lofftus, Paul Berdahl, and
Param Tewari experimented, looking for safer preparations and
processes.

One known alternate compound favored by Russo was
tetraethylorthosilicate (TEOS). However, the only aerogels ever
made with TEOS were less transparent and more shrunken than the
aerogels made with TMOS. The LBL group conducted a number of
experiments with TEOS and focused on the base catalysis process.
Ultimately, they tried ammonium fluoride, an acid catalyst. Voila,
the result was a clearer aerogel and less shrinkage.

Sven Henning, one of the few other scientists in world then
doing aerogel work, was visiting Hunt's group in 1984 when word
arrived that his small aerogel manufacturing facility in Sweden,
the world's first, had exploded. Gases escaping the autoclave
aerogel drying apparatus had ignited, blown the roof off the plant,
demolished the building, and injured several employees who were
hospitalized.

Hunt was motivated to explore alternate aerogel drying
processes.

The drying process in use at the time relied on alcohol. When
the gel was ready for drying, it was loaded into a pressure
vessel, alcohol was added, and heat was applied. At 280 C and 1800
pounds per square inch of pressure, the alcohol was a supercritical
fluid. After reaching that plateau, pressure was slowly released
and the supercritical alcohol gradually was vented from the vessel.

In addition to the evident potential for explosions, Hunt
realized that this process was too costly for successful
commercialization. The high pressures and temperatures required
massive, expensive fabrication chambers, and beyond that, it was an
energy-hungry process. Hunt looked for a substitute for alcohol.
The surrogate substance had to become supercritical at a lower
temperature and pressure and it had to be nonflammable.

Liquid carbon dioxide proved to be the ideal supercritical
fluid. Under pressure, it becomes liquid at near room temperature.
And whereas alcohol can be bomb-like, carbon dioxide is fire-
quenching. Hunt's carbon-dioxide aerogel drying process has been
patented.

From scratch, Hunt's aerogel process begins with the mixing
of TEOS and water. To allow these two immiscible fluids to loosen
up and mix, alcohol is added. The water breaks apart the TEOS,
attacking the silicon bonds, and creating an intermediate ester
that condenses into pure silica particles. With the assist of a
catalyst, ammonium fluoride, and a solution of ammonium hydroxide
to control the pH, the silica particles grow and link, forming an
alcogel. A clear gel, the alcogel is sufficiently strong so that
when a bottle is half filled with it and turned upside down, it
will not flow.

The gel is then inserted into a pressure vessel where liquid
carbon dioxide flushes out and replaces the alcohol in the gel,
reducing potential fire risks in the process. Pressure is
increased, the carbon dioxide becomes supercritical, and as it is
slowly vented, the alcogel dries into an aerogel.

Hunt says he continues to fine-tune the drying process. "In
principle," he says, "the carbon dioxide process is straight-
forward, but you have to practice the process. At 600-800 pounds
per square inch, there are a whole world of things going on inside
that pressure vessel. It's like driving a sports car on a mountain
road. You have to slow down, speed up, make adjustments in the
pressure and temperature. You can crack-up in a car and you can
fracture aerogels or, you can make them crack-free."

With these multiple refinements, Hunt and company had created
a safer, more energy-efficient process that required less massive
and costly equipment. He turned next to the problem of clarity.

Aerogels were transparent but they were not transparent enough
to be used in double-paned windows. They scatter light through a
natural process first described by Lord Rayleigh in the late 19th
century. This phenomenon -- Rayleigh scattering -- is why the sky
looks blue against the dark background of outer space and why the
same sky looks yellow when viewed in the direction of a setting
sun. Hunt's aerogels scatter light in a similar manner. Placed
against a dark background, they appear bluish whereas against a
light background, they are yellowish.

Hunt decided to tweak his recipe, altering the quantities of
the five compounds that go into the gel plus the variable of
temperature in an effort to increase clarity. Some 500 formulations
were tested and additional variations were evaluated using a
powerful experimental technique called factorial design analysis
that helps pinpoint the roles that different ingredients play.

Additionally, Hunt drew on his doctoral thesis work, employing
a beloved, mothballed device he had devised to measure light
scattering. The scientist retrieved his trusty, old scanning
polarization modulated nephelometer. The nephelometer measured
several of the 16 separate elements of the light scattering matrix
of various experimental formulations of aerogels, allowing Hunt to
isolate and identify the structures responsible for scattering.

"My measurements revealed that the largest of the pores was
responsible for the scattering and the haziness in aerogels. The
cross-linked silica particles are extremely fine, 20-40 angstroms
in diameter. That is smaller than the wavelengths of visible light
and too small to cause scattering, which is good news. The average
pore size was 200 angstroms but the largest in our TEOS gels were
3,000 angstroms. The large pores are the problem."

By filtering out impurities in the starting solution,
improving the overall cleanliness of operations, and providing more
uniform gelling conditions, pores larger than 500 angstroms have
been eliminated. This has considerably improved the clarity of
Hunt's aerogels, making them suitable for use in skylights or
atrium coverings. But further research and development is necessary
before aerogels are totally transparent. Until then, the promise of
aerogel-insulated double-paned windows will remain just out of
sight.

On the other hand, aerogels could make their debut as
insulation in refrigerators within several years.

Refrigerators and freezers account for about 20 percent of
residential electricity use in the U.S. Because of a vast potential
for energy savings through the use of available, cost-effective
technology, Congress passed the National Appliance Energy
Conservation Act in 1987. Implementing the act, the Department of
Energy (DOE) has announced rules requiring improved energy
efficiencies in appliances with the new standards for refrigerators
taking effect January 1, 1993. Only a handful of the 2,000 models
now on the American market meet the 1993 standards. The DOE has
pledged it will impose yet more stringent standards in the future
as soon as new and affordable technology makes this practical. (The
1993 DOE standards are based on technical and economic analysis
performed by Isaac Turial and Jim McMahon in LBL's Applied Science
Division.)

Every year in the U.S., 300 million square feet of insulation
are used in new refrigerators. Currently, the insulation of choice
is polyurethane foam which is expanded into refrigerator walls by
chlorofluorocarbons (CFCs). Refrigerators account for an estimated
two percent of the U.S.' annual CFC usage. The U.S., through an
international treaty and the Clean Air Act of 1990, has committed
to halt its production of ozone-destroying CFCs by the end of the
century.

Three insulating materials, all silica-based, are the leading
candidates to replace foams. The competition pits aerogels against
silica powder and glass beads which are sealed inside steel sheets.
All three insulating systems would be sealed in a partial vacuum to
increase their thermal resistance.

In a partial vacuum, aerogels outperform silica powder and
glass beads. Inch-thick aerogels have the same R value (a measure
of thermal resistance) as inch-thick foams. But when 90 percent of
the air is evacuated from a plastic-sealed aerogel packet, the R-7
value nearly triples to R-20 per inch. To match the R-value of
aerogels at this vacuum of one-tenth of an atmosphere, silica
powder has to be evacuated to a few thousandths of an atmosphere.
Glass beads require one-billionth of an atmosphere.

Achieving a vacuum of one-tenth of an atmosphere and
sustaining it for the lifetime of a refrigerator is a piece of
cake. Existing plastic vacuum packing techniques can do the job.
Maintaining a vacuum of one-thousandth of an atmosphere or better
is a major technological challenge.

Whereas Hunt doesn't have to worry about vacuum sealant
technology, he is under pressure to reduce the cost of aerogels.
About $20 of foam goes into a 1991 model refrigerator using 40-
square-feet of polyurethane insulation. Insulating the same
refrigerator with aerogels would cost in excess of $80. The
aerogels, however, would have double the R-value of foam and in two
years, the energy saved would recoup the $60 in additional costs.

Hunt has conducted fundamental studies on how heat is
transmitted through aerogels in an effort to improve the material.
The less aerogel necessary for a given application, the lower the
cost.

Research shows that the little remaining heat which is
conducted through an aerogel under vacuum is attributable to solid
conduction through the silica lattice and to radiant heat transfer.
The solid and the radiative component each account for about half
of the heat that passes. Focusing on neutralizing the radiative
element, Hunt conducted analysis which pinpointed the spectra of
infrared energy which aerogels conduct. Whereas aerogels block the
passage of most wavelengths, they are transparent to infrared
radiation between the wavelengths of three and eight microns.

Hunt began a Cinderella-like search for an additive that would
block the infrared energy in this wavelength region. The substance
had to fit the job at hand and no less than a perfect fit would do.
The perfectly-proportioned additive would absorb infrared radiation
in the three to eight micron region, be available in small particle
sizes, not interfere with the gelation or drying process, disperse
uniformly without clumping, and be non-toxic and inexpensive.

All these years later, Hunt remains entranced by aerogels.
They were the best solid insulator known when he first saw them and
he has made them even more impervious to heat. Today, Hunt
continues to work on improving aerogels. Currently, he is
contriving to fabricate them using still less raw material so that
they are yet cheaper and lighter, just a wisp of solid within a
filigree of air. Beautiful as Hunt finds the new aerogels, the
scientist intends to create ever more elegant aerogels, materials
that consumers and manufacturers will find absolutely irresistible.